The present invention relates generally to optical laser systems, and more particularly toward a heterodyne laser receiver having folded Gregorian, all-reflective collecting optics.
It has been proposed that communication between data relay satellites as well as between satellites and spacecraft be provided optically using data-modulated laser beams. Each satellite and spacecraft is equipped with a receiver comprising an infrared laser beam detector, and signal beam collecting optics. An incoming laser beam in the infrared region, i.e., having a wavelength of approximately 10 microns, received by the beam collecting optics, is mixed (heterodyned) with a local oscillator beam generated by an auxiliary laser, and the resultant is impinged on a beam detector. The difference between the wavelength of the incoming signal beam and the wavelength of the local oscillator beam is process to extract the data contained in the signal beam.
Coarse pointing of the collecting optics is provided with a gimbal-mounted pointing mirror. For fine pointing, however, image motion compensators are included in the optical path of the collector to align any off-axis incoming signal beam to the beam detector. Image motion compensation is best performed at or very close to the exit pupil of the collector, and, for maximum detection efficiency, must direct the incoming beam to the detector coaxially with the local oscillator beam. This requires that off-axis as well as on-axis inputs be aligned to the optical axis of the collector at the auxiliary laser.
It has been suggested that Cassegrain collecting optics comprising a paraboloid main reflector confocally oriented with respect to a hyperboloid subreflector be used in conjunction with a laser beam detector. The exit pupil in the Cassegrain configuration, however, is virtual rather than real. The image motion compensator, accordingly, cannot be located within the Cassegrain configuration without the addition of a refractive optical relay system. It is undesirable to include a refractive lens in the system, because a refractive lens is wavelength dependent. This is a serious disadvantage because data relay satellites generally operate at two discrete frequencies (diplexing).
Furthermore, the alignment of the reflectors is very critical to the operation of the system. The collector is preferably tested by supplying light to the collecting optics, and visually monitoring the response. A collector having a refractive lens would have to be tested at the wavelength of actual operation, e.g., infrared for a 10.6 micron wavelength communication system, and it would be impossible to visually determine alignment. It is desirable that the collecting optics be frequency independent so that visible light can be used for testing.